1. Field of the Invention
This invention relates generally to a crystal controlled oscillator, and more particularly, to an oscillator in which multiple crystals forming a portion of the feedback circuit are connected with the oscillator sustaining stage to provide an improved reduction in phase noise sideband levels and a decrease in the oscillator long term frequency instability. A preselected wavelength section of coaxial cable may be used to connect the multiple crystals and the sustaining stage when physical separation between the crystals and the sustaining stage is necessary.
2. Background Information
Crystal controlled oscillators are well known in the art and generally comprise a single crystal resonator and associated feedback circuitry coupled across a sustaining stage such as an appropriately biased transistor amplifier. For example, U.S. Pat. No. 4,550,293 discloses a narrow band, voltage controlled crystal oscillator having a linear frequency versus tuning voltage response. The oscillator utilizes a single crystal to form a portion of a composite resonator network, and it is stated that the composite network provides improved oscillator output signal frequency stability.
Crystal controlled oscillators which employ a pair of crystal resonators to achieve various results are also well known. U.S. Pat. No. 3,836,873 discloses a two transistor VHF crystal controlled harmonic oscillator having a circuit configuration which provides a large ratio of output power to crystal unit power dissipation without significant degradation of oscillating resonator Q from the crystal unit Q. A cascode amplifier configuration comprising first and second transistors in combination with inductance and capacitance elements provides an oscillator configuration with phase shift in the vicinity of the oscillator frequency dominantly controlled by the quartz crystal unit motional impedance parameters. It is stated that either one or two VHF crystal units with appropriate anti-resonating circuitry may be utilized to achieve a desired minimum of phase noise power spectral density in the oscillator circuit.
U.S. Pat. No. 4,570,132 discloses an oscillator circuit in which two quartz crystal resonators are utilized for frequency control. It is stated that the use of two SC-cut crystal units increases the oscillator circuit signal transmission group delay by a factor of two to three and effects an estimated 6 dB reduction in the oscillator output signal phase noise sideband level at carrier offset frequencies less than the resonator half-bandwidths.
U.S. Pat. No. 4,575,690 discloses a crystal oscillator which includes two crystals connected in either series or parallel relationship with each other. It is stated that the crystals have unequal acceleration sensitivity magnitudes, and are mounted such that their respective acceleration sensitivity vectors are aligned in anti-parallel relationship. At least one electrical reactance, such as a variable capacitor, is coupled to one of the crystals for providing cancellation of acceleration sensitivities. Although the oscillator circuits in this patent illustrate two crystals connected in either series or parallel relationship, U.S. Pat. No. 4,575,690 does not teach the use of two crystals in an oscillator circuit to provide reduced levels of crystal-induced low frequency noise, vibration sensitivity and circuit temperature rise as set forth herein.
While each of these prior art devices includes either a single crystal or a pair of crystals to perform various functions such as establish the oscillator operating frequency and reduce the oscillator output signal phase noise sideband level, there is a need for an improved crystal controlled oscillator capable of providing greater reductions in output signal phase noise sideband levels and long term frequency instability than crystal controlled oscillators heretofore utilized. If desired, the oscillator must also be capable of operating in high vibration environments without experiencing the vibration-induced frequency modulation problems that conventional crystal oscillators would normally experience in this situation. Finally, the oscillator must be designed to prevent the crystals from being subjected to the heat generated by the oscillator active stage power dissipating elements.